>perl
my $s = 'abc;  ;  ;  ;def';
$s =~ s/;\s*;/;/g;
print "RE1", $s, $/;
$s = 'abc;  ;  ;  ;def';
$s =~ s/(?:;\s*)*;/;/;
print "RE2", $s, $/;
^Z
RE1abc;  ;def
RE2abc;def